The accumulation of damaged or misfolded proteins can have devastating effects on cellular physiology and viability. To protect itself, the cell possesses numerous Protein Quality Control (PQC) systems that minimize the persistence and detrimental effects of these aberrant proteins. While a variety of PQC systems have been characterized in the cytoplasm, ER, and mitochondria, how the nucleus manages aberrant proteins is poorly understood. But, understanding this fundamental aspect of nuclear biology is important because many age-correlated neuromuscular disorders (Huntington's, Kennedy's, several spinal-cerebellar ataxias) result from toxic accumulation of aberrant proteins in the nucleus. Therefore, our long-term goals are to understand how the nucleus normally protects itself from toxic aberrant proteins by identifying PQC systems that operate in the nucleus, characterizing how these systems recognize and target aberrant proteins, examining if lesions in nuclear PQC systems are required for these pathologies to develop, and determining the specific nuclear processes that are disrupted by aberrant nuclear proteins to cause toxicity. From our initial studies in S. cerevisiae, we've identified the first-known PQC degradation system that acts in the nucleus. The central player of this nuclear PQC system is San1, a nuclear-localized ubiquitin- protein ligase that targets aberrant nuclear proteins for ubiquitination and proteasome degradation. With the discovery of San1, we're now well positioned to explore how a nuclear PQC degradation system recognizes its substrates, and what it recognizes as aberrant within those substrates. The specific aims for this grant are to: (1) Determine the regions within San1 responsible for substrate targeting. Mutagenesis of San1 will be used to identify in cis regions important for substrate recognition. (2) Determine the features of an aberrant protein recognized by San1. Mutagenesis of San1 substrates will be used to identify regions required for targeting by San1. (3) Identify & characterize additional components of the San1 pathway. A combination of genetic and biochemical approaches will be used to identify additional San1 pathway components.